1. Field of the Invention
The present invention relates in general to aircraft gas turbine engines requiring jet exhaust vectoring in all directions for enhanced aircraft maneuverability and pertains especially to such engines requiring temperature suppression if exhaust nozzle flowpath surfaces.
2. Description of Prior Developments
Fighter aircraft must be highly maneuverable to survive attack by modern tracking missiles and sophisticated anti-aircraft defense systems. Current fighter aircraft maneuverability is limited by the excess available power of the engine and the maximum angle of attack that the wings can sustain without experiencing buffet and stall. In-flight thrust vectoring by the engine exhaust system can alleviate these conditions and thereby improve the maneuverability and survivability of an aircraft fighter.
Many types of thrust vectoring nozzles have been studied during the past 20 years. Thrust vectoring generally falls into two categories covering two basic approaches. The first category is divergent vectoring, wherein the jet exhaust is deflected by actuation of the divergent nozzle flaps so as to deflect the jet exhaust while the convergent nozzle accelerates the flow continuously in an axial, nonvectored direction. The second category is primary vectoring, which varies the orientation of the entire exhaust nozzle so that the exhaust flow is accelerated at a desired vector angle.
The major advantages of divergent vectoring are minimum transverse excursion during vectoring and low weight. A limitation of this concept is the difficulty in providing adequate divergent nozzle flowpath cooling due to high divergent wall gas pressures. The exhaust gas which flows against the divergent nozzle flowpath surfaces during thrust vectoring generates high impact pressures which must be overcome by highly pressurized bypass cooling air flowing through the flowpath surfaces and into the exhaust gas flowstream.
An important advantage of primary vectoring is the relatively low divergent wall gas pressures and uniform circumferential gas pressure distribution which exists since the flaps do not deflect the flow in vectoring. This allows for not only simple low pressure air cooling of the flow path surfaces but, in addition, for more effective temperature suppression of the exhaust nozzle. Improved temperature control can be provided by the introduction of low pressure and low temperature ram air through cooling slots provided along the full length of the divergent flowpath surface. A limitation of primary vectoring has been excessive exhaust nozzle weight.
A common primary vectoring approach for accomplishing the required multi-directional pivoting of the exhaust nozzle uses a pair of gimbal rings which function much the same as the gimbal system commonly used for mounting navigation compasses. Although these gimbal rings are very effective in providing the kinematic needs of nozzle vectoring, the extremely high loads applied to the gimbal rings by the exhaust nozzle impose a large weight impact on the thrust vectoring system. The axial and transverse forces applied by the exhaust gas to the exhaust nozzle during vectoring are high, for example, on the order of approximately 40,000 lbs. and 7,000 lbs., respectively.
While these forces are easily handled when distributed on a conventional exhaust duct, when they are concentrated in two orthogonally related gimbal rings, the flexing and twisting moments on the rings together with the local reinforcement required for concentrating and redistributing the loads results in an excessive added structural weight requirement. The additional weight required to provide vectoring capability using the gimbal ring approach can reach 40% to 60% of the basil nozzle weight. Moreover, in some cases, gimbal designs have had to be eliminated because of the nonacceptable deformations of the gimbal rings.
Gimbal rings, when designed to handle the applied loads, are not only heavy but they also tend to exceed the aircraft contours at the interface between the nozzle and the aircraft fairing, wing or body. This is a serious aerodynamic drawback since any significant deviation in the preferred contours leads to increased afterbody drag with attendant reduction in aircraft range.
In addition to reacting the transverse force applied on the exhaust duct by the primary vectoring nozzle, the greater axial force must be reacted in a manner which is consistent with the pivoting action of the nozzle, which is low in friction and which is low in weight. An alternative design is therefore required which will essentially perform the function of the previously used gimbal rings but at a much lower weight, at a high vectoring rate and without exceeding the preferred aerodynamic contours at the interface between the nozzle and the aircraft.
Accordingly, a need exists for a vectorable nozzle which requires much less added weight than nozzles which incorporate gimbal rings and which is compatible with the low drag aircraft contours at the nozzle-fairing interface. Such a nozzle should be capable of effective temperature suppression of the nozzle flowpath surfaces and require minimum transverse excursion during vectoring for aircraft installation compatibility.
A further need exists for such a nozzle which has a high vectoring rate capability and exhibits high reliability and safety. Still another need exists for such a nozzle which uses low pressure ram air for cooling the nozzle flaps, particularly the divergent flaps of a convergent-divergent nozzle.